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An earlier underwater breathing regulator, known as the régulateur, was invented in France in 1860 by Benoît Rouquayrol. He first conceived it as a device to help assist in escaping from flooded mines. The Rouquayrol regulator was adapted to diving in 1864, when Rouquayrol met the lieutenant de vaisseauAuguste Denayrouze. The Rouquayrol-Denayrouze apparatus went into mass production and commercialization on 28 August 1865, when the French Navy Minister ordered the first apparatuses.[3]

After 1884, several companies and entrepreneurs bought or inherited the patent and produced it until 1965. In 1942, during the German occupation of France, the patent was held by the Bernard Piel Company (Établissements Bernard Piel).[4] One of their apparatuses went to Émile Gagnan, an engineer employed by the Air Liquide company. Gagnan miniaturized and adapted it to gas generators in response to a fuel shortage, which was a consequence of German requisitioning. Gagnan's boss, Henri Melchior, knew that his son-in-law Jacques-Yves Cousteau was looking for an automatic demand regulator to increase the useful period of the underwater breathing apparatus invented by Commander Yves le Prieur,[5] so he introduced Cousteau to Gagnan in December 1942. On Cousteau's initiative, the Gagnan's regulator was adapted to diving, and the new Cousteau-Gagnan patent was registered some weeks later in 1943.[6] After the war, in 1946, both men founded La Spirotechnique (as a division of Air Liquide) in order to mass-produce and sell their invention, this time under a new 1945 patent, and known as CG45 ("C" for Cousteau, "G" for Gagnan and "45" for 1945). This same CG45 regulator, produced for more than ten years and commercialized in France as of 1946, was the first to actually be called the "Aqua-Lung". In France, the terms scaphandre autonome ("scuba set"), scaphandre Cousteau-Gagnan ("Cousteau-Gagnan scuba set"), or CG45 were meaningful enough for commercialization, but to sell his invention in English-speaking countries, Cousteau needed an appealing name following English language standards. He then coined the trade name Aqua-Lung.

In the late 1940s and early 1950s, La Spirotechnique started exporting the Aqua-Lung and leasing its patent to foreign companies (like the British Siebe Gorman). These operations found great success. The Rouquayrol-Denayrouze apparatus didn't achieve as much success because the compressed-air tanks made with the technology of the time could only hold 30 atmospheres, which allowed dives of only 30 minutes at no more than ten meters deep.[7] Before 1945, French divers preferred their then traditional diving helmets and diving dresses. When the Aqua-Lung became available for commercial use, divers around the world found a scuba device smaller and easier to carry than its precursor, which in fact was almost completely unknown outside of France. In addition, and most importantly, the Aqua-Lung could be mounted on stronger and reliable air tanks holding up to 200 atmospheres,[8] allowing extension of diving duration to more than an hour at significant depths (including the needed time for decompression stops).

The first Cousteau-Gagnan Aqua-Lungs (like the CG45 of 1945 or the Mistral of 1955) were mainly twin-hose open-circuit scuba. They have since been made by various manufacturers with varying design details and numbers of cylinders. Like modern open-circuit scuba with single-hose regulators, they consisted of one or more high pressure diving cylinders and a diving regulator (the Aqua-Lung) that supplied the diver with breathing gas at ambient pressure via a demand valve. For more than ten years, seen in the films Épaves (Shipwrecks, 1943) and Le Monde du silence (The Silent World, 1956) the main scuba equipment used by Cousteau and his divers was an Aqua-Lung mounted on three diving cylinders, one being used as a safe air reserve.[9]

The original "Aqua-Lung" was an "open-circuit" design (so-called because gas flows from the cylinder, to the diver, out into the water). Other scuba systems that were invented before the "Aqua-Lung" were "closed circuit" (or "rebreather"). In these apparatuses, breathing gas flows from the cylinder to the diver, through a scrubber (which removes carbon dioxide), back to a secondary bag or counter-lung, and then back to the diver again, in a closed loop.

The Aqua-Lung was not the first breathing apparatus, but it was the most popular. In 1934, René Commeinhes developed a firefighter's breathing apparatus that was adapted by his son Georges to diving in 1937. It was used by the French Navy during the first few years of World War II. The twin-hose or double hose Aqua-Lung "demand regulator" forms the foundation of all modern scuba regulators. A diaphragm is used to deliver the breathing gas to the diver on demand, at ambient water pressure.

The first modern two stage, single hose regulator was manufactured and distributed as the "Sport Diver" by Divers Supply in Wilmington California beginning in 1951. This design, an offshoot of surface supplied commercial gear, uses a first stage regulator mounted at the tank valve delivering air to a mouthpiece-mounted second stage via a small-diameter intermediate-pressure (about 140 psi) hose.

At about the same time as Divers Supply began selling the Sport Diver regulator, Australian Ted Eldred designed a similar single hose system which he marketed as the Porpoise. Today, virtually all modern open-circuit SCUBA gear uses this design, though Aqualung did market a "modern" double hose Mistral model in 2005 and 2006.

There is controversy over the invention of the first single hose regulator. The invention was motivated by an effort to bypass Aqua-Lung's patent on the double hose regulator. This patent involved the return of exhaust gas to the regulator to reduce the differential pressure and therefore reduce work of breathing. The single hose regulator accomplishes this by relocating a portion of the regulator to the point of exhaust, rather than routing the exhaust back to the regulator.

Divers Supply, which had been supplying commercial surface-supplied breathing systems, replaced the surface air supply with a tank-mounted first stage pressure regulator. Ted Elred claims to have developed his nearly identical system contemporaneously in Australia. Since neither party chose to patent the invention, it is difficult now to know if either design was inspired by the other or if, in fact, they were truly spontaneous inventions. There seems to be some evidence that the Divers Supply unit hit the market just prior to the Elred Porpoise, but there is no evidence that Elred was aware of it.

In the early years of scuba diving in Britain, "tadpole" was a nickname for a type of diving gear that had two meanings:

A type of ex-Royal Air Force pilot's oxygen cylinder that had a tapering end, and was often used as an aqualung cylinder in the 1960s and earlier.

An early make of Siebe Gorman aqualung with a twin-hose regulator and two air cylinders. Both ends were hemispherical, and were 13 inches long and 7 inches in diameter. Siebe Gorman's trade catalog describing this set showed two sorts of diver wearing this set, both with weighted boots, and with no mention of free-swimming. A 1950s Royal Navy diving manual also said that the Aqua-Lung was only for bottom-walking diving. Siebe Gorman either had no idea about sport diving then, or else was against sport diving, and expected aqualungs to be used for light commercial diving. Later most recreational divers were free-swimming scuba divers, with bottom-walkers virtually restricted to those doing commercial work.

Aqualung, Aqua-Lung, and Aqua Lung are registered trademarks for scuba diving breathing equipment. That trade name is owned in the United States by the firm formerly known as U.S. Divers. The term was in use before the trademark was registered by Rene Bussoz, who owned a sporting goods store called Rene Sports in Los Angeles. He obtained a contract with the French firm L'Air Liquide to import the new scuba equipment into the US for sale on the Pacific coast (SPACO had the contract for the Atlantic coast). Bussoz changed the name of his company to US Divers and registered the name Aqua-Lung. This turned out to be a wise move, because when the French company decided not to renew his 5 year contract, no one had of even heard of their product, but everyone was familiar with the names he had registered. He sold the company and the trade names for a handsome profit, returning to France. The name US Divers sounded very official and very American, but it was owned by a Frenchman and sold to a French company.

L'Air Liquide held the patent on the original "Aqualung" (also written as Aqua-Lung or Aqua Lung) until the patent expired sometime around 1960 to 1963. The term "Aqualung", as far as is known, first appeared in print on page 3 of Jacques-Yves Cousteau's first book, The Silent World, in 1953. Public use of the word "Aqualung", and public interest in Aqualungs and scuba diving, were started around 1953 in English-speaking counties by a National Geographical Society Magazine article about Cousteau's underwater archaeological expedition to Grand Congloué. In France, Aqualung diving was popularized by Cousteau's movie Épaves, while his book The Silent World also helped significantly.

As with some other registered trademarks, the term "aqualung" became a genericized trademark in English-speaking countries as a result of common use by the public and in publications, including the BSAC's official diving manuals. Presumably, lawyers for Cousteau or L'Air Liquide could have slowed or stopped this genericization by taking prompt action, but this seems not to have been done in Britain, where Siebe Gorman held the British rights to both the trade name and the patent.

In the United States, the term Aqualung was popularized by the popular television series Sea Hunt (1958). This series never said that an aqualung could be called anything else, or made by anyone else, but the company that supplied (the fearless) Mike Nelson. The Voit Rubber Corporation provided most of the diving equipment used in this series, but actual Aqua-Lungs appeared in early episodes. The word "aqualung" was commonly used in speech and in publications as a term for an open-circuit, demand valve-controlled breathing apparatus (even after Air Liquide's patent expired and other manufacturers started making identical equipment), occasionally also for rebreathers, and in figurative uses (such as "the water spider's aqualung of air bubbles"). The word entered the Russian language as the generic noun акваланг ("akvalang"). That word was taken into Lithuanian as the generic noun "akvalangas"; "langas" happens to be Lithuanian for "window", giving a literal meaning "aqua-window".

In the United States, US Divers managed to keep "Aqualung" as a trademark. The acronym "SCUBA", or "Self Contained Underwater Breathing Apparatus", originated in the United States Navy, where it meant a frogman's rebreather. Scuba became the generic term for that type of open-circuit breathing set, and soon the acronym SCUBA became a noun — "scuba" — all in lower-case. "Scuba" was a trademark for a time – used by Healthways, now known as Scubapro - one of the competitors of US Divers.

In Britain Siebe Gorman (who held the rights to the tradename "Aqualung") made no serious attempt to control use of the word, and "aqualung" remained a common public generic word for that sort of apparatus - including in the British Sub-Aqua Club's official publications – for many years.

^Both regulators—the one from 1860 invented by Benoît Rouquayrol and the twin-hose Cousteau-type invented in 1943 by Gagnan and Cousteau—received, among some others, the name of régulateur (French for "regulator"). For the 1860 régulateursee the page of the Rouquayrol-Denayrouze apparatus in the Musée du Scaphandre website (a diving museum in Espalion, south of France). For the word régulateur as used by Cousteau himself just check page 8 in the first French edition of Cousteau's book The Silent World: Jacques-Yves Cousteau & Frédéric Dumas, Le Monde du silence, Éditions de Paris, Paris, 1953, Dépôt légal 1er Trimestre 1954 – Édition N° 228 – Impression N° 741 (in French).